In manufacturing an electronic device such as, for example, a fin type field effect transistor, a plasma etching is performed so as to transfer a pattern of a mask to a silicon layer. In some cases, since a natural oxide film is formed on an object to be processed (“workpiece”) including the silicon layer and the mask, the plasma etching of the silicon layer may be performed after removing the natural oxide film. Typically, the natural oxide film is removed by exposing the workpiece to the plasma of a fluorocarbon-based gas.
When removing an oxide film such as the natural oxide film, it is required to selectively remove the oxide film with respect to the silicon layer and the mask. A method called chemical oxide removal (COR) is known as a method for selectively removing the oxide film. In the COR, the workpiece is exposed to a processing gas including hydrogen, nitrogen, and fluorine. As a result, silicon oxide forming the oxide film is denatured to (NH4)2SiF6, i.e., ammonium fluorosilicate, thereby forming a denatured region. Subsequently, in the COR, the workpiece is heated so that the ammonium fluorosilicate of the converted region is thermally decomposed. Therefore, the oxide film is removed.
In some cases, the denatured region may also be formed by a plasma processing. That is, the denatured region may be formed when the workpiece is exposed to the plasma of the processing gas. The COR using plasma is disclosed in Japanese Laid-Open Patent Publication No. H6-188226.